Last updated: April 26, 2013
1987: YACs Developed
Yeast artificial chromosomes (YAC) can carry large segments of DNA from other species, like humans.
A YAC is copied reliably when a yeast cell divides. Plasmids - pieces of DNA in bacteria that can carry foreign DNA - and viruses are also used in the lab to store and copy fragments of human DNA. But YACs can carry million base-pair-long fragments of human DNA, whereas plasmids and viruses carry a few thousand base-pair-long pieces only. Being able to deal with bigger fragments of DNA made mapping the human genome easier, especially when scientists still had relatively few landmarks to navigate by.